The measurement of bore hole diameters and other geometric properties of cavities has always been a challenge. An early approach has been the use of plug gages, where a plug of the proper diameter is inserted in the bore. If the plug is too loose, the bore is too big and if the plug is too tight, the bore is too small.
A more quantitative measure of the bore is provided by air gaging. To measure a bore hole, two jets of air flow out from an air gage in opposite directions toward the wall of the bore. The back pressure measurement is accurate only for a very small gap between the air jet and the wall of the bore. Therefore, a specific air gage is only good for a very short range of bore diameters. Also, the gage only measures two opposite points on the bore. If information is required around the entire bore, the gage must be rotated. To operate well, a bore gage requires clean pressurized air.
Recently, electronic gages have replaced some of the air gages for bore hole measurement. With electronic gaging, a position sensitive electronic probe replaces each of the air jets. Electronic probes are not as compact as air jets so the minimum diameter for electronic gaging is limited. Another drawback of electronic probes is that they require contact to make a measurement which can result in wear and breakage.
Another approach for measuring bore diameter is to use a coordinate measuring machine with a contact trigger probe. The probe is inserted into the bore, and then moved perpendicular to the bore's surface until contact is made. This is repeated until sufficient data is available to calculate the bore diameter and other desired bore properties. Using a coordinate measuring machine with a contact trigger probe, the measurement process is slow and requires a costly machine.
The U.S. patent to Harding U.S. Pat. No. 4,875,777 discloses an optical gauging system for evaluating surface shape of a workpiece along a cross-section utilizing an offaxis, high accuracy structured light profiler.
An article by Harding and Bieman the inventors hereof, entitled "Position Decoupled Optical Inspection Relay System" Volume 1194, pp. 28-35 of "Optics, Illumination, and Image Sensing for Machine Vision IV", SPIE, Nov. 8-10/ 1989, Philadelphia, Pa. discloses a relay optical system on a machine in conjunction with a mechanically decoupled imaging system on a camera.
An article by Harding one of the inventors hereof, entitled "Analysis of Methods for Image Rearrangement" Volume 2065, pp. 10-of "Optics, Illumination, and Image Sensing for Machine Vision VIII", SPIE, Sep. 8-9/ 1993; Boston, Ma. discloses the use of multiple lenses along the length of a part whose images are fed to a large collection lens.